Proceedings of the Fifth International WLT-Conference on Lasers in Manufacturing 2009
Munich, June 2009
High-power Yb-fibr...
Hence, the need for a shorter processing time and for a
documentable real-time process monitoring was
identified.
In order...
Fig. 2: process head, with industrial vision system in
position. The vision system and Precitec welding head
are mounted o...
Fig. 4: simplified schematic of the welding system’s communications and control logic.
Once the coordinate system of the w...
Fig. 5: visualisation of vision system algorithm result.
If filler wire is used in the process, the system
will start feed...
3 Production trials and results
The complete system has undergone long rigorous
testing from the point of view of all hard...
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Qnet: fully automatic robotic laser welding station

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Design and implementation of a fully automatic robotic laser welding station by Qnet.
Here is the relative technical article published at the Proceedings of the Fifth International WLT Conference on Lasers in Manufactoring 2009 in Munich by Davide Kleiner and Geert Verhaeghe

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Qnet: fully automatic robotic laser welding station

  1. 1. Proceedings of the Fifth International WLT-Conference on Lasers in Manufacturing 2009 Munich, June 2009 High-power Yb-fibre laser welding of heavy-section tube-to-tubesheet assemblies Davide Kleiner1 , Geert Verhaeghe2 1 Mangiarotti S.p.A., Research & Development Department, Udine, Italy 2 TWI ltd., Laser and Sheet Processes Group, Cambridge, UK Abstract Mangiarotti S.p.A., a leading pressure vessel, reactor and heat exchanger manufacturer, have developed, with the help of TWI Ltd, a unique fully-integrated laser processing cell for the welding of heavy-section tube-to-tubesheet components. Laser welding was identified as a high- productivity alternative to the conventionally applied TIG welding process, offering speeds at least an order of magnitude higher. The welding cell is fully autonomous, can be moved around the workshop, and is capable of welding heat exchangers typically comprising up to 5000 tubes and tubesheet diameters in excess of 4.5m. An 8kW Yb-fibre laser is manipulated using a 6-axis shelf- mounted articulated robot arm, and a machine vision operated slide to ensure accurate tube location and joint alignment. Filler wire addition and scanning optics have been implemented to compensate for joint gap variations, with real-time (laser) process monitoring to assist in assuring a weld quality in accordance with relevant product standards (e.g. ASME). Keywords: pressure vessel, tube, tubesheet, weaving, mechanised 1 Introduction and background Mangiarotti S.p.A. is based in the North-East of Italy and operates in the field of pressure, high temperature as well as heavy-wall and large diameter equipment engineering and manufacturing. Its typical products comprise, among others, heat exchangers, condensers, reactors, columns, finishers, drums, separators and splitters. Production materials include stainless steels (solid and clad), non-ferrous metals (e.g. nickel-based alloys), carbon steels, alloyed steels (e.g. Cr, Mo and V) and reactive metals, such as titanium. Mangiarotti operates in the industry fields of oil&gas (onshore and offshore), chemical, petrochemical, nuclear, pharmaceutical and power generation, and utilises in its work all the relevant construction codes such as ASME, TEMA, API, PD5500, PED, AD Merkblätter, Stoomvezen and RCC-M. The large pressurised items that Mangiarotti manufactures, such as tubular reactors or heat exchangers, comprise tube-to-tubesheet welding. This is traditionally carried out using the tungsten inert gas (TIG) process with the addition of suitably chosen filler wire. This is a long proven technique, amply discussed and regulated in the codes relevant to Mangiarotti’s applications, and is currently carried out in-house in either manual, semi-automatic (orbital machines) or fully automatic (CN controlled) modes, depending on the specific application and associated welding procedures. Because in most cases the tube-to-tubesheet weld is to be a full-strength weld, the TIG procedure requires chamfering of the tubesheet holes and multipass welding with the addition of filler wire to achieve the desired geometrical weld features, above all the minimum length of the weld throat. The welding time per tube depends on the component geometry, but can currently take in excess of one minute. Considering a heat exchanger can easily comprise a total of 5000 tubes, the total welding time is considerable and constitutes a large percentage of the total manufacturing time of a heat exchanger. Moreover, the heat input for welding this type of joint associated with a typical TIG procedure can reach values in the order of 40MJ/m2 on the tubesheet surface. As a consequence of this, weld metal solidification stresses and deformation of the tubesheet are always a concern regardless of its thickness. A great deal of attention is also given to the quality control of the process. This is however traditionally limited to the establishment of a thoroughly tested welding procedure developed on test samples, trial welds on test samples performed at the beginning of each work shift. Ultimately, the TIG welding of the tube-to-tubesheet joint is a blind or almost blind process, until thorough non-destructive testing of the fully welded tubesheet is performed.
  2. 2. Hence, the need for a shorter processing time and for a documentable real-time process monitoring was identified. In order to stay ahead of its competitors, the Directorate at Mangiarotti decided to investigate alternative manufacturing methods that would allow them to reduce the total processing time per heat exchanger, whilst maintaining the required weld quality through process monitoring. In 2006, Mangiarotti decided to explore the benefits and assess the feasibility of laser beam welding as an alternative to TIG welding for the welding of tube-to-tubesheet. The Research & Development Department, with the cooperation of the Production and Quality Departments, carry out some initial studies on the aforementioned welding technique on full-size mockups made of carbon, low alloy and stainless steels. Despite the first welding results being far from perfect, the technique showed to be very promising in terms of repeatability and productivity, and it was decided to proceed with a full project, which would take laser beam welding into the production environment. In early 2007, Mangiarotti called upon the specific expertise of TWI ltd (Cambridge, UK) to assists them in designing a dedicated laser facility for welding their tube-to-tubesheet products. 2 System design 2.1 Philosophy From the very start of the conceptual design, a number of key characteristics were identified as being crucial to the successful completion and operation of the welding system. These comprised constraints and boundary conditions from both the product and the process point of view, as illustrated below. - Properties of the weld must be acceptable according to the relevant codes and standards. This includes the surface appearance, the cross-sectional weld features, the presence of imperfections and the local and overall mechanical qualities. - The cycle time, tubesheet deformation and joint reject rate must be less than that of equivalent TIG welded joints. - Strict quality control procedures must govern the welding process, and the quality control on the individual joints must be fully traceable. - The system must be capable of interpreting the data from all the on board sensors and take operational decisions based on that interpretation. The system must evaluate the reliability of this decision before the start of any welding. - The process must be easy to set up, fully automated be capable of unsupervised operation. - The envelope within which the system parameters are flexible must be sufficiently large not to be a constraint during welding procedure development. - The system software must be user friendly and must be able to directly address all the welding parameters of the system hardware. - The safety and emergency subsystems must prevent the occurrence of any situation that could prove potentially hazardous to the operator or any other person. 2.2 Hardware The result of the development is an integrated system mounted on a transportable steel structure, linked to a control unit located in a lightproof safety bunker. The completed tube-to-tubesheet welding system is shown in Fig. 1. Fig. 1: processing end of the completed welding system. The laser source is an 8kW Yb-fibre laser manufactured by IPG Photonics. The laser, with a nominal beam quality of 6mm.mrad, was selected over alternative options, because of Mangiarotti's requirement to be able to move the laser system around the welding shop. The source comprises a built-in 2- way beam switch transmitting the optical power through a 30m long 200µm core fibre to a modified YW-50 Precitec process head, capable of focusing the power into a 0.40mm minimum beam waist, using a 150/300mm lens combination. The Precitec process head is equipped with a set of in-line process monitoring sensors and a scanner unit, manufactured by ILV, that by employing oscillating optics achieves laser beam weaving, which can be precisely modulated both in frequency and in amplitude. The process head, together with a vision system and hotwire torch are mounted on the wrist of a shelf-mounted GE Fanuc 6-axis robot (Fig. 2 and Fig. 3) with an extended reach of 3.5m and a useful wrist payload of 100kg. The vision system comprises an industrial camera equipped with a high quality Sony CCD and purposely chosen lenses, and a set of two stripe laser diodes, controlled also by the camera itself. The hotwire torch is part of a complete Fronius digital welding set and mechanised wire-feeder unit, all mounted on the robot arm.
  3. 3. Fig. 2: process head, with industrial vision system in position. The vision system and Precitec welding head are mounted on pneumatic slides and are toggled depending on whether the system is performing a centring or a welding operation. Fig. 3: process head, with industrial vision system in position. The setup of the mechanical stops on the pneumatic slides is such that the axis of the CCD corresponds to axis number 6 of the robot. Controls and communication protocols between the various subsystems take place via analogue, digital I/O, Ethernet, Profibus and Canbus, all handled by Beckhoff PLC units. Simplified schematics of the communications and control logic that run the whole welding system is shown in Fig. 4. Festo equipment is employed for the management of all shielding gas and compressed air circuits, using, where possible, fireproof pressure tubing and connecting joints. A dedicated air compressor feeds all the welding system’s compressed air lines. The whole installation is located inside a laserlight-proof welding cell of 20x61m, controlled from an annexed safety bunker, where the control console is located, and accessories, such as warning indications, emergency and door interlock circuits. The control console contains the industrial pc which handles the system software (including the user interface), various plc controls and the provision of power for the on-board low voltage subsystems. The machine is equipped with two on-board cameras, one wall-mounted PTZ camera and one special applications camera coaxial to the laser beam axis. The feed from all of these four cameras is conveyed to a PC in the safety bunker and recorded on a Milestone video server. The PTZ camera can also be accessed from any PC within Mangiarotti by use of a username and password. 2.3 Process cycle dynamics As with any automated mechanical cycle, the process starts with the precise identification of the position and orientation of the workpiece in 3D space. The coordinate system of the workpiece is specified with respect to the robot’s own internal coordinate system by means of an X,Y,Z translation of the origin and P,Q,R rotations around the X,Y,Z axes respectively. The three points which are necessary to create a coordinate system in 3D space (in this case origin, X- direction and Y-direction) are identified by the robot’s vision system itself. The vision system software performs auto- setup operations using Montecarlo algorithms for exposure brightness and contrast optimisation, acting on the camera shutter time and aperture. It identifies the tube by applying mathematical image processing algorithms so as to enhance the geometrical feature of the tube’s inner circumference. An example of this is shown in Fig. 5. The numbers that can be seen in the same figure are the outputs in terms of positioning error in the X and Y directions. These are then communicated to the robot, which carries out a repositioning. The maximum allowed error in both of these directions is 0.05mm. This is smaller than the robot’s intrinsic repeatability, hence this cycle is repeated until convergence to within that value occurs, which generally takes two or three cycles. The distance between the welding head and the tubesheet is measured by means of two stripe laser diodes projected on the tubesheet in front of the industrial camera. Being the focal length 300mm, the tolerance in the Z position is slightly larger, and is set to 0.2mm.
  4. 4. Fig. 4: simplified schematic of the welding system’s communications and control logic. Once the coordinate system of the workpiece has been acquired by the robot, the automated welding cycle can start. The robot positions itself in front of the tube to be welded at the theoretical X,Y,Z position according to the CAD/CAM information, aligning the wrist (and hence the welding head) parallel to the tubesheet. A fine centring cycle is performed until the positional tolerances are met. The system toggles a set of two pneumatic slides, moving the vision system out of the way (and closing an external protection shutter) and the welding head into the right position. The radius of welding around the tube axis is determined by the position of a mechanical stop mounted on the slides. The position of this stop is regulated during the development of the welding parameters. The activation of the crossjet, the shielding gas and the oscillating optics follows. Shielding gas can be fed via the welding head coaxial nozzle, the side nozzle of the wire-feeder torch, or both. The circular movement required for welding is provided by the rotation of axis number 6 of the robot; no other axis moves during the welding. The laser source starts emitting as the head starts revolving, with a preset ramp up time lasting 5° of head rotation. The system then welds at constant power during 365° of head rotation followed by a power ramp down lasting 12° of head rotation, for a total of 382°. The crossjet, the shielding gas and the oscillating optics are at this point deactivated. Control system and human interface Milestone videoserver E Bunker Welding cell Company network E PTZ camera E 2x onboard cameras Welding head camera E Laser source and chiller unit A Door interlocks Emergency circuits Flashing lights g/y/r 6-axis robot D P Wire feeder Oscillating optics control Welding headQA sensors control Pneumatics and shielding gas control A A Pneumatic and gas actuators A Legend: A: analog C: canbus D: digital E: ethernet P: profibus D Machine vision system C
  5. 5. Fig. 5: visualisation of vision system algorithm result. If filler wire is used in the process, the system will start feeding it at a rotation angle determined by the user (>0°), and retracting it at the end of the constant power section (370°). Before the system proceeds to the following tube, the system checks whether the filler wire has become stuck in the solidified weld pool. 2.4 Software The software was developed externally by Qnet s.r.l. in Visual C# .NET and runs on the industrial pc in the control console. The software is, generally speaking, the master in the system except while the robot is carrying out a movement, during which it acts as a slave. The software contains a CAD/CAM module as shown in Fig. 6, so that dxf or dwg files can be uploaded directly, while the system parameters that can all be addressed by the software are shown in Fig. 7. Fig. 6: CAD/CAM interface and automated welding process commands. 2.5 Quality control The software structure is that, if there is any doubt about the reliability of the sensors’ reading, the system will not weld. Indeed, the machine monitors continuously all the cycle parameters such as head position with respect to tube being welded, real tube position with respect to theoretical tube position, real tube diameter with respect to theoretical tube diameter, tubesheet surface deviation from flat, welding head position with respect to machine vision assembly, shielding gas flow, compressed air static and dynamic pressure, laser beam weaving parameters, wire speed voltage and current, torch position, rotation speed of axis number 6 of the robot, wire stuck to the weld, operating temperature of the system subcomponents and their three separate cooling circuits, and status of the access doors to the welding cell. Any abnormal reading in any of the aforementioned parameters instructs the machine to skip the tube in question or terminates the process altogether. Furthermore, the welding head contains sensors that monitor directly the status of the weld pool at 1kHz frequency, and more specifically the molten metal temperature, the reflected laser light, the plasma status over the weld pool and the effective laser power output. This data, together with the wire speed voltage and current, rotation speed of axis number 6 of the robot and images used by the machine vision system, is collected live and recorded. The data from the welding sensors is then compared against “acceptability curves” created during welding of the mockups. Any deviation from the optimum readings gives rise to an error message and stops the process. All the monitoring data is recorded for post-weld analysis, and any noticeable event that is “cause for concern” is communicated to the operator via SMS. Fig. 7: welding parameters selection. Where possible, more than one sensor monitors a specific system. This is especially true for the machine vision system, which is critical for the correct positioning of the welding head in relation to the centre of the tube. All critical subsystems dealing with system status and information exchange and management are provided with battery backup. A very rigid structure exists for data management on the control console pc. Welding parameters, their revision number, CAD/CAM data (and their association with the welding parameters), machine vision images, sensors’ readings etcetera are given unique IDs for traceability reasons.
  6. 6. 3 Production trials and results The complete system has undergone long rigorous testing from the point of view of all hardware, software and welds produced. It is immediately clear that mechanised laser welding (or cutting) is an intrinsically highly repeatable process. All tubes welded on the same tubesheet with the same parameters look identical both visually and in cross-section. Fig. 8 shows a typical surface finish for a stainless steel joint, with its microsection shown in Fig. 9. Fig. 8: surface finish of a typical autogenous stainless steel joint (Ø19.05x1.65mm 321 tube to 347 tubesheet). Fig. 9: microsection of the same joint. The effect of the scanning optics on the weld profile is very visible. The last two figures represent one of two configurations that have been so far successfully completed with a WPS (welding procedure specification) in accordance to the ASME code. Initial pull-out destructive tests have performed three times better than what was required in the joint specification (60-65kN against 20kN), and no defects where detected by non-destructive testing. 4 Comparison against equivalent TIG tube-to-tubesheet welding procedures The system is currently occupied in the development of welding procedures for the welding of real assemblies, to be compared with the traditionally used TIG procedures. The data obtained from the testing of both the laser and the TIG samples will provide the grounds for further validation of the system and subsequent full integration into production on the shop floor. The welding procedures developed so far exhibit a cycle time of between 50 and 10% that of equivalent TIG welding. Furthermore, there is a time saving on the tubesheet preparation as well, as the joint requires no chamfer. The resulting heat input of the laser beam welding process is approximately 10% of that of equivalent TIG welding. As a result, deformation of the tubesheet after welding and asymmetries of the tube in the tube opening are minimal. The resulting chemical composition and hence the metallurgical properties of the weld metal have been successfully controlled through the application of small quantities of appropriate filler wire, generally for non-stainless materials. 5 Conclusion Validation work on the system continues for the tube- to-tubesheet welds, as new materials, material grades, joint geometries or mechanical requirements are investigated. At the time of this article, Mangiarotti had started publicising this welding system with its clients, a number of which responded positively asking for further information and for a live demonstration. Welding procedure specifications have already been produced for the product of a specific client, which is being manufactured at Mangiarotti. More unusual materials have also been successfully welded under the ongoing research programme, such as for example 99.9% pure copper to 70/30 CuNi alloy.

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